The invention relates to four-stroke cycle internal combustion engines, particularly to an exhaust gas temperature management method to optimize exhaust emission quality.
Contemporary internal combustion engines for use in automotive vehicle drivelines may use a three-way catalytic converter in the exhaust gas flow circuit for the purpose of reducing carbon monoxide, unburned hydrocarbons and oxides of nitrogen. To improve NOx removal, it is known design practice to use an additional three-way catalytic converter, often referred to as a lean NOx trap (LNT), at the exhaust flow outlet side of the upstream three-way catalytic converter. At periodic intervals, the lean NOx trap and upstream three-way catalyst can be purged by momentarily enriching the fuel mixture to produce excess carbon monoxide and hydrocarbons, which react with the trapped NOx to produce nitrogen and carbon dioxide. The time interval for operating in the rich mode is very short compared to the LNT total operating time.
The temperature of exhaust gases passing through the catalytic converter and the lean NOx trap must be controlled so that the reactions taking place in the presence of catalysts will have optimum efficiency. Optimum NOx emission control is obtained when LNT temperature is in a predetermined temperature range; for example, 250xc2x0 C. to 450xc2x0 C. Under low load operating conditions, however, the three-way catalyst and the lean NOx trap temperatures may be too low for efficient operation.
A known method for generating additional heat of exhaust gases includes generating additional unburned hydrocarbons, which will react with oxygen in the three-way catalytic converter and release additional heat. The additional unburned hydrocarbons are generated in some known systems by directly injecting fuel into the combustion chamber during the exhaust stroke of the combustion cycle. This fuel does not create added torque, but it does partially disassociate in the exhaust stroke when it encounters hot exhaust gas. This added fuel is not burned in the combustion chamber, but it provides excess hydrocarbon content to the exhaust gases, which must be oxidized by excess air to generate additional heat.
Stratified charge engines, which are known in the automobile industry, include fuel injectors that inject fuel directly into the combustion chamber of the engine during the compression stroke when operating in the stratified mode. The injected fuel mixes with the air inducted from an air intake manifold through one or a pair of intake valves to create a mixture that is relatively rich near the ignition point and within the ignition kernel, but the air/fuel ratio becomes progressively more lean as the stratified charge distribution travels through the combustion chamber to regions where the mixture becomes too lean to support combustion. The average air/fuel ratio for the entire stratified charge, however, may be outside the ignitable range of 20:1 to 40:1, although the air/fuel ratio at the kernel is in the ignitable range of about 12:1 to 16:1.
The lean portion of the stratified charge that does not burn is expelled through the exhaust valve during the exhaust stroke of the combustion cycle. This unburned mix is combined with the mix created by the direct injection of fuel into the combustion chamber during the exhaust stroke so that sufficient oxidation or burning of the hydrocarbons may take place in the catalytic converter and, therefore, increase the temperature in the lean NOx trap.
The invention comprises a method for controlling hydrocarbon content of exhaust gases of a four-stroke cycle internal combustion engine with spark ignition, the engine having at least one piston and cylinder assembly with an air/fuel combustion chamber. A fuel injector delivers timed fuel injections to the combustion chamber. An ignitor ignites an air/fuel charge in the combustion chamber.
The method includes the steps of injecting fuel during the piston intake stroke as air is drawn into the combustion chamber to form a non-combustible homogeneous mixture, compressing the mixture, injecting fuel also during the piston compression stroke, igniting the stratified charge that is created near the end of the compression stroke and discharging the combustion gases and the non-combustible gases during the piston exhaust stroke, thereby creating excess hydrocarbons in an exhaust gas manifold system.
An embodiment of the invention comprises a method for creating in the combustion chamber a homogeneous air/fuel mixture in addition to a stratified charge resulting from a later injection. The homogeneous mixture is too lean to burn in a homogeneous mode. This lean mixture, which is deliberately produced in a non-combustible air/fuel ratio range, is created by injecting fuel into the combustion chamber during the intake stroke rather than in the compression stroke, as in the case of a conventional stratified charge engine. The charge formed during the intake stroke may have a lean air/fuel ratio such as 30:1 to 40:1. This occurs in advance of the subsequent injection of fuel for stratified charge operation.
The fuel is injected during the intake stroke sufficiently in advance of the closing of the intake valve so that there is enough time for the air and fuel to achieve a homogeneous mix. In a subsequent fuel injection event, a stratified charge is created in the combustion chamber during the compression stroke. Because the air/fuel ratio of the homogeneous mix is too high to support combustion, the homogeneous mix will produce unburned hydrocarbons in the combustion chamber, which are exhausted through the exhaust valve during the exhaust stroke.
The injection of fuel during the intake stroke does not result in an extension of the lean limit for the stratified air/fuel charge. Rather, the injection of fuel during the intake stroke is intended to deliberately obtain a non-extension of the lean limit.
Excess hydrocarbons may be produced in an alternative fashion by controlling the timing of the fuel injection and the spark ignition when the engine is operating in the stratified mode. Unlike conventional stratified charge operations, for which the timings of the injection and the ignition are adjusted to create maximum torque and minimum exhaust gas emissions, the timing characteristic of the present invention is deliberately adjusted so that the fuel is injected earlier than normal. This achieves a lower maximum torque but higher hydrocarbon production. This increase in the hydrocarbon component of exhaust gases provides the necessary excess hydrocarbons to effect optimum temperature for efficient operation of the lean NOx trap.
The difference in the timing between the injection and the ignition can be achieved by appropriate spark timing adjustment or by fuel injection timing adjustment, or by a combination of both techniques.
Because it is possible for the engine to run with an overall leaner air/fuel mix by applying the teachings of the invention, it is not necessary to rely upon the throttle to the extent required by conventional stratified charge engines. The engine, therefore, may be operated with a more open throttle than the throttle setting normally associated with a conventional engine for the same engine load. This reduces the pumping losses during the intake stroke of the four-stroke cycle.
A further benefit that results from the use of a homogeneous mix created by the injected fuel during the intake stroke is the improvement in thermal efficiency. The invention makes it possible to retain the usual benefits of stratified operation. This is due to the insulating effect of the lean homogeneous mix. The homogeneous mix creates a thermal barrier between the combustible part of the stratified charge and the walls of the combustion chamber during the expansion stroke.